1. Field of Invention
The present invention relates generally to an inkjet head to be used for an inkjet type printer or the like, more specifically to an inkjet head having piezoelectric device to apply positive pressure or negative pressure to ink to control ink ejection.
2. Description of the Related Art
FIG.1 shows the configuration of a conventional inkjet head. In such an inkjet head, an ink passage 103 and a plurality of ink chambers 105 (one of which is depicted) are formed on a plate portion 104. Ink is supplied from the ink passage 103 to the ink chambers 105, and then, a piezoelectric element 101 applies pressure to the ink, so that the ink is ejected through an ink nozzle 102.
The ink ejecting action of the inkjet head is further explained with reference to FIG. 2. FIG. 2 is a sectional view taken substantially along the line II—II of FIG. 1. As shown in FIG. 2, electrodes 106 to be negatively charged and electrodes 107 to be positively charged are mounted on the piezoelectric element 101. These electrodes 106 and 107 are located at positions corresponding to the ink chambers 105, respectively. By using the electrodes 106 and 107, an electric field whose direction is perpendicular to the polarization direction of the piezoelectric element 101 is applied to the piezoelectric element 101 in order to distort the piezoelectric element 101 in a shear mode (thickness shear mode). Namely, when the electric field is applied, the piezoelectric element 101 deflects inwardly into the ink chambers 105, and pressure is applied to the ink within the ink chambers 105. In such a manner, the ink ejection can be realized. In addition, the shear mode means a mode of distortion that occurs by applying an electric field whose direction is perpendicular to the polarization direction of a piezoelectric element.
Furthermore, an ink ejecting action of another inkjet head using the shear mode is explained with reference to FIG. 3. FIG. 3 is a sectional view of the inkjet head. As shown in FIG. 3, electrodes 202 to be positively charged and electrodes 203 to be negatively charged are mounted on a piezoelectric element 201. The electrodes 202 are located at positions corresponding to ink chambers 205 formed on a plate portion 204, respectively. The electrodes 203 are located at positions corresponding to side wall portions, respectively. By using the electrodes 202 and 203, an electric field whose direction is perpendicular to the polarization direction of the piezoelectric element 201 is applied to the piezoelectric element 201 in order to distort the piezoelectric element 201 in the shear mode. Thus, it is possible to apply pressure to ink within the ink chambers 205 to eject the ink.
Moreover, an ink ejecting action of an inkjet head using a distortion mode other than the shear mode is explained with reference to FIG. 4. FIG. 4 is a sectional view of the inkjet head. As shown in FIG. 4, a diaphragm 303 is disposed on ink chambers 304 formed on a plate portion 305 as an upper wall. Laminated piezoelectric members are mounted on the surface of the diaphragm 303. Each of the laminated piezoelectric members is formed of a plurality of piezoelectric elements 301 and a plurality of electrode layers 302. The piezoelectric elements 301 and the electrode layers 302 are alternately laminated. By using the electrode layers 302, an electric field whose direction is parallel to the polarization direction of the piezoelectric elements 301 is applied to the piezoelectric elements 301 in order to distort the piezoelectric elements 301 in an expansion mode (longitudinal vibration mode). Thus, it is possible to apply pressure to ink within the ink chambers 304 to eject the ink. In addition, the expansion mode means a mode of distortion that occurs by applying an electric field whose direction is parallel to the polarization direction of a piezoelectric element.
However, in the inkjet head using the shear mode shown in FIG. 2 or FIG. 3, distortion of the piezoelectric element 101 or 201 is relatively small. Therefore, it is required to apply a high voltage to the piezoelectric element 101 or 201 in order to obtain sufficient distortion to realize optimum ink ejection.
On the other hand, in the inkjet head using the expansion mode shown in FIG. 4, distortion of the each laminated piezoelectric member is relatively large. Therefore, sufficient distortion can be obtained by a low voltage. However, the diaphragm 303 is made of a soft elastic material so as not to restrict distortion of each piezoelectric element 301. Therefore, pressure which has been applied to ink within the ink chambers 304 is reduced due to the soft elasticity of the diaphragm 303.
Furthermore, in the inkjet head using the expansion mode shown in FIG. 4, the plate portion 305, the diaphragm 303, the piezoelectric elements 301, and so on are separated as independent parts, respectively. When assembling the inkjet head, these parts are accurately bonded to each other at the predetermined positions by using adhesive. Therefore, if the size of an inkjet head is reduced, the manufacture of the inkjet head is difficult.